[docs][vue-demo][minor] add ID in div element (#506)

Author: pratu16x7

docs/vue.md

@@ -42,7 +42,7 @@ You can manually initialize a Vue instance.
 ```markdown
 # Vue demo
 
-<div>hello {{ msg }}</div>
+<div id="main">hello {{ msg }}</div>
 
 <script>
   new Vue({